The present invention relates generally to chemical mechanical polishing. More particularly, the present invention relates to an apparatus and method for determining the thickness loss of a polishing pad due to pad conditioning.
Chemical mechanical polishing (CMP) is a commonly used operation for polishing substrates in the semiconductor manufacturing industry. CMP may be used to planarize substrates, or to remove a deposited film from a substrate by polishing. A CMP operation may be used multiple times during the sequence of process operations used to form a semiconductor device. CMP is available for polishing any of various films used to form semiconductor devices. Many commercially produced CMP tools are available in the semiconductor manufacturing industry.
The polishing pad is generally considered to be the major component of a CMP tool. The polishing pad is generally rotated while contacting the surface to be polished, thereby polishing the surface. The CMP operation must perform a precise and accurate polishing operation. It can be understood that it is important to maintain the characteristics of the polishing pad in the same condition to ensure repeatability and integrity of the polishing operations. In order to maintain the pad characteristics in the same condition and therefore to keep film removal rates constant, the hard, upper section of the polishing pad which includes the polishing surface, requires periodic dressing with a diamond conditioner in order to maintain the integrity and repeatability of the upper section of the polishing pad, especially the polishing surface. As a result of this necessary conditioning process, however, the pad thickness decreases with a corresponding decrease in planarization ability. In addition to the decrease in planarization ability, a conditioned pad of reduced thickness also effectuates other undesirable process non-uniformities, and adversely affects run-to-run repeatability. When this occurs, the polishing pad must be replaced. There is a need to determine the extent of polishing pad thickness reduction, in order to predict when polishing characteristics may be adversely affected and, therefore, when the polishing pad must be replaced.
The standard approach to determining thickness loss and remaining pad thickness consists of cutting out a radial piece of the pad, peeling off the bottom soft pad and using a micrometer to measure the thickness of the hard upper pad directly. This is a destructive test, however, and once used, renders the pad unusable. This is the case even if the measured thickness indicates that the condition of the pad would be acceptable for future use. The standard approach is therefore time consuming and costly. Even if a correlation between [# of conditioning operations] and [pad thickness loss] is established to predict when to replace the polishing pad, the actual thickness loss can vary from pad to pad, in practice.
It can therefore be seen that a method and apparatus for measuring pad thickness, which is non-destructive, is needed in the art. To overcome the shortcomings of the conventional approach to determining polishing pad thickness, the present invention provides an apparatus and method for measuring polishing pad thickness loss, which is non-destructive and, furthermore, does not require the polishing pad to be removed from the polishing tool.
To achieve these and other objects, and in view of the its purposes, the present invention is directed to a method and apparatus for performing an in-situ measurement of polishing pad thickness loss. Specifically, the present invention provides a rigid first planar member which is placed over a non-conditioned portion of the polishing pad, and a rigid second planar member which is placed over a depressed, conditioned portion of the polishing pad. The present invention further provides a movable upper member positioned over the top of the first planar member, and which includes an overhang portion which overhangs an upper surface of the second planar member. The upper member includes at least one measurement instrument such as a thickness gauge, which measures a vertical distance between respective upper surfaces of the first and second planar members. The vertical distance, or height difference, is due to thickness loss caused by the conditioning process on the conditioned portion of the polishing pad. The upper member may be repositioned and further measurements taken, in order to calculate an average polishing pad thickness loss.